The present invention relates generally to a method for remotely measuring the atmospheric variables used for weather prediction and more particularly to a method for estimating the refractivity profile of the Earth's atmosphere.
Measurements of satellite systems such as Global Navigation Satellite Systems (GNSS) are generally accurate, however, they operate on frequencies that are sensitive to atmospheric effects. Similar satellite-based navigation systems such as GLONASS and GALILEO are also sensitive to atmospheric effects. To enable the GNSS systems to detect any changes in atmospheric properties or any slight change in refraction of the signals due to natural variations in the atmosphere, sensitive receivers must be used. Currently GNSS systems rely on phase shifting calculations to measure atmospheric properties. To produce weather predictions and forecasts, the GNSS systems measure the excess phase shift induced by the GNSS signals following a refracted path through the atmosphere to a GNSS receiver, rather than the straight path the GNSS signal would follow if there were no change in the atmospheric properties. As the GNSS satellites rise or set, the length of the path that the GNSS signals travel through the atmosphere varies due to refraction. The amount of refraction varies based on how much change there is in the atmospheric properties. Also, as the signal path length and atmosphere refractivity vary, the phase shift of the GNSS signals change as well. Currently, in order to generate refractivity profiles from phase measurements taken along various lines of sight, various algorithms are used with data gathered through phase shifting measurements. Most methods currently measure phase shift directly, which requires advance knowledge of the location of the receiving antenna. When the receiving antenna is carried on a moving platform such as an aircraft, determining the precise location of the antenna makes calculating the phase shift even more difficult.
Another current system for weather prediction measures atmospheric changes using measurements of excess Doppler shift versus time, and then uses these measurements to estimate the phase shift. This method requires less knowledge of the precise location of the antennas and receivers; however, it has yet to be initiated in real-world applications. All of the prior and current solutions for measuring atmospheric refractivity changes to predict weather forecasts are generally characterized by having poor signal-to-noise ratios. Because of the poor signal to noise ratios, the excess phase shift caused by any changes in temperature or humidity approaches the resolution limit for even advanced GNSS receivers. GNSS receivers that are in motion, including the receivers moving on non-ballistic paths, are especially affected by poor signal-to-noise ratios. Even the Doppler shift versus time approach has a similar problem with the poor signal-to-noise ratio. The excess Doppler shift due to temperature or humidity variations in the atmosphere is close to the frequency resolution limit for receivers on mobile platforms.
Another deficiency with the current systems is that they operate as if the atmosphere is horizontally homogeneous. While incorrect, this assumption is required for the refractivity profile algorithms used in these systems. While the assumption of a horizontally homogeneous atmosphere is the best solution for this system, it leads to errors in refractivity estimates and degrades the horizontal resolution of occultation measurements, thereby creating errors in weather prediction based on these methods.
Thus, there is a need for a method and system to accurately measure refraction of GNSS signals caused by changes in the Earth's atmosphere. With such a system, more accurate GNSS measurements can be recorded, and further, more accurate weather predictions will result.